1. Field of Invention
The present invention relates to the growth of silicon crystals according to the Czochralski method, and more particularly to improvements to pulling processes and related equipment for growing silicon crystals having high and controlled carbon content.
2. Background Art
The present trend im microelectronics towards highly integrated ssemiconductor devices requires more and more a good knowledge of the crystalline structure of the substrates used. In general, these substrates result from the slicing of monocrystalline silicon cyrstals obtained from the Czochralski pulling technique.
It is generally known that the presence of oxygen atoms in the silicon substrate is benefical. These atoms incorporated into the crystal during the pulling process tend to diffuse during high temperature processing and to cluster, providing both valuable hardening and gettering effects. Hardening is a mechanical strength enhancement due to the fact that oxygen precipitates impede dislocations and slips. Gettering is due to high microstresses at the viciniity of oxygen precipitates, which allow the capture of fast diffusing impurities, mainlly metallic impurities. U.S. Pat. No. 4,220,483, assigned to the same assignee as of the present invention, describes in detail the gettering phenomemon.
Recently, it has been demonstrated that carbon atoms, also incorporated in silicon crystals during the crystal growth, can play a significant role in the oxygen precipitation kinetics as well, when the substrates are submitted to subsequent thermal treatments. The carbon incorporation in silicon crystals is assumed to occur from carbon monoxide (CO) absorption by the melt. Since the carbon vapor pressure is extremely low, once a CO molecule is trapped by the melt, carbon will stay and become incorporated later on into solid silicon according to the freezing segregation coefficient.
FIG. 1 shows a schematic cut view of conventional prior art crystal puller equipment of the atmospheric pressure type. A monocrystalline rod-shaped silicon crystal 11 is grown from a silicon melt 12 contained in a quartz liner 13. The crucible 14 consists of a rotating graphite holder, surrounded by an electric resistor 15. Other heating means may be used. The seed 17 is affixed on a rotating mandrel 18. The pulling device, per se, is enclosed in a chamber 19, which is continuously filled with a neutral gas such as argon. Seals 20 and 21 are necessary at crystal and crucible rotation shaft apertures to prevent leakage. In addition, the inside pressure is slightly above atmospheric pressure to avoid contamination.
Thermochemical reactions that are likely to occur in pullers, to produce CO, are understood to be as follows:
reactions between silica crucible and graphite holder: EQU C.sub.s +SiO2.sub.s .revreaction.SiO.sub.g +CO.sub.g ( 1) EQU 3C.sub.s +SiO2.sub.s .revreaction.SiC.sub.s +2CO.sub.g ( 2)
reactions between SiO and graphite: EQU SiO.sub.g +2C.sub.s .revreaction.SiC.sub.s +CO.sub.g ( 3)
reactions between graphite and oxygen: EQU C.sub.s +O2.sub.g .fwdarw.CO/112.sub.g ( 4) EQU C.sub.s +CO2.sub.g .revreaction.2CO.sub.g ( 5)
The subscripts s, l, g denote solid, liquid and gaseous phases respectively.
It is to be understood that uncontrolled CO generation may give rise to silicon carbide production via reaction: EQU 2Si.sub.s or l +CO.sub.g .revreaction.SiC.sub.s +SiO.sub.g ( 6)
Solid silicon carbide particles, if produced by reaction between the silicon melt and the carbon monoxide over the melt, can be driven by melt flow towards the freezing interface. Due to the presence of these defect particles, an interface breakdown occurs and is followed by the formation of polycrystalline grains. In such a case, also called structure loss, the crystal has to be remelted to be pulled again in good condition. Should such a rework operation not be possible, the crystal would have to be considered a reject for integrated circuit manufacturing purposes.
For background literature on this topic, reference is made to an article by F. Schmid et al., J. Electrochem. Soc. 126.6 (1979). There has been a view in the art as taught, for example, in the IBM Technical Disclosure Bulletin, Vol. 25, No. 4, Sept. 1982, pages 1905-1906 in an article entitled "Low Carbon Czochralski Crystal Growth" by R. C. Guggenheim et al., that carbon is an undesirable impurity, and care should be exercised to reduce, as much as possible, the carbon content in the grown silicon crystals.
However, now there is a strong tendency to consider that carbon atoms are beneficial to the gettering phenomenon, in cooperation with oxygen atoms. A good example of that approach may be found in European Patent application No. 84109528.4, entitled "Silicon Wafer and its Application in Producing Integrated Circuit Devices", by D. C. Ahlgren et al., and assigned to the same assignee as of the present invention. Disclosed in this patent application is a silicon wafer material, having a high carbon concentration in the range between 0.5 ppma and the solid solubility limit of carbon in silicon, and a low oxygen concentration in the range between about 15 ppma and 28 ppma. The wafer is capable of providing improved integrated circuit devices with significantly improved yields, due to large defect-free zones having a high quality crystal lattice formed therein during device fabrication.
Another aspect disclosed in Ahlgren et al. is a manufacturing process of sorting the wafers formed from a melt-grown silicon crystal, by characterizing the wafers. The process is comprised of: measuring the concentrations of carbon and silicon therein and grouping the wafers into classes in accordance with their oxygen and carbon concentrations. The wafers having high carbon and low oxygen concentrations are further processed through a device production line for obtaining high yield and high quality integrated circuit devices. According to the teachings of Ahlgren et al., once the silicon crystal rod has been pulled, the wafers are obtained from dicing the rod as standard, then characterized and individually sorted by measuring their concentration of carbon. It may happen, that only a small portion of the rod has the desired carbon concentration, resulting in a waste of costly wafers. In addition, the suggested technique is complex and long. However, this application has the advantage of clearly emphasizing the key role of carbon on the final product yields. As it may be understood from Table I of Ahlgren et al., with wafers having the same oxygen concentration, (e.g. 31 ppmA), the percentage of wafers having a leakage current of less than 20 pA raises from 23% to an astonishing 73% when the carbon content slightly raises from about 1 to 3 ppmA.
The principle of introducing small amounts of carbon to Czochralski-grown silicon crystals is known, as described in the IBM Technical Disclosure Bulletin, Vol 25, No. 3A, August 1982, pages 962-963, an article entitled "Increasing Carbon Content in Czochralski Grown Crystals" by D. C. Ahlgren et al. In that TDB no means is suggested on how to obtain controlled and reproducible carbon content. The secondary effects of introducing oxygen into the chamber on the oxygen content in the silicon crystal are not analyzed. Lastly, according to that process, there is no indication on the actual amount of carbon introduced in the silicon crystal. In other words there is no control of carbon incorporation in the silicon crystal.
Japanese Patent No. 59-18191 by Shigeo Enoki, owned by Shinetsu Handoutai K. K., teaches a method of reacting a carbon containing gas with polycrystalline raw material during melting. Particularly, it is shown that a fixed amount of CO gas is introduced at the top of the furnace (i.e., chamber) and is reacted with the silicon melt. However, there are numerous problems associated with this method.
The Enoki patent does not teach a method or means for obtaining predictable, reproducible carbon content in silicon crystals. The method disclosed does not provide for real time in-situ control that changes as the kinetics of the process change, but merely provides for volumetric control of a carbon containing gas. No predictable and reproducible correlation is made between the amount of carbon containing gas introduced and the ultimate carbon content in the silicon crystal in part because there is no real time in situ control and in part because of the kinetics, wherein not all of the CO present in the puller atmosphere is trapped by the melt, but only the fraction over the melt surface. This fraction depends on gas vortices. The gas vortex itself is a function of thermal distribution depending on heating and packaging geometry. Since it is not possible to exactly duplicate the geometry and consequently the vortex run after run on the same puller or from one puller to another, a wide spread of carbon content in the melt would be expected. In addition, any excess carbon introduced into the chamber during the method becomes an unwanted dopant. Due to the presence of this unwanted dopant, the flow rate of the carbon containing gas introduced into the chamber would have to be recalibrated routinely. Applicants have in fact experimented with introducing carbon containing gases into the chamber atmosphere. Specifically, methane was introduced, and the resultant process had no control over the final content of carbon in the crystal. It is believed that the equilibrium kinetics between the carbon introduced from the outside and the carbon preexisting in the chamber was erratic.
In addition, very low yields resulted when methane was introduced because the monocrystalline structure of the silicon was destroyed. This can be explained by the fact that adding excess carbon into the chamber atmosphere increases the partial pressure of carbon gas in the chamber, causing the formation of excess silicon carbide. When silicon carbide gets into the melt, it causes defects in the crystal lattice, thus making the resultant crystal polycrystalline. The excess silicon carbide also adheres to parts inside the chamber, resulting in increased maintenance time required to clean silicon carbide from the chamber and parts therein.
Thus, the major drawbacks of utilizing Enoki to control carbon content in silicon are that there is no in-situ control apparatus or method disclosed which during the pulling process would be able to consistently predict the ultimate carbon content in a crystal; the kinetics are hard to control when carbon from outside the puller reacts with carbon preexisting therein; and low yields and increased maintenance requirements due to excess silicon carbide that is formed.
In a recent publication, Y. Endo et al (J. Electrochem. Soc. 126.8.1979) proposed a combined oxygen-carbon control in silicon crystals by means of CO monitoring during crystal pulling.
They assumed reactions (1) and (6) to occur, and an added reaction: EQU CO.sub.g +SiO.sub.g .revreaction.SiO.sub.g +C (7)
They also defined an apparent equilibrium constant K. ##EQU1## [C].sub.si and [O/11].sub.si being oxygen and carbon content in the silicon crystal at a location in the crystal rod corresponding to the time the carbon monoxide concentration was measured. Without any theoretical justification, K was found to be close to the equilibrium constant of reaction: EQU C.sub.si +O.sub.si .revreaction.CO.sub.g ( 8)
derived from different experiments and computations. Experiments carried out by the inventors of the present invention, using Endo et al approach, concluded that no relationship could be established between carbon monoxide concentration [CO], measured during crystal pulling, and the corresponding product of the carbon content and oxygen concentration in silicon, [C].sub.si .times.[O].sub.si, for well defined locations in crystal rod, although [CO] values are in the same range of 10 to 50 ppmA observed by Endo et al. This is demonstrated in FIGS. 2A to 2E. In addition to that, no correlation could be made between oxygen present in a molecular state in the puller atmosphere, given by the oxygen concentration (O2), and oxygen concentration in silicon: [O].sub.si (see FIG. 2F).
We can explain the failure of the Endo approach by the fact that carbon and oxygen are incorporated into a silicon crystal through completely separate independent mechanisms. It is generally admitted that oxygen incorporation in silicon results from silicon crucible dissolution in the silicon melt via reactions: EQU SiO2.sub.s .revreaction.Si.sub.l +2{O} (9)
where {O} denotes dissolved oxygen in the silicon melt. Part of this oxygen is incorporated into the growing solid, while the most important fraction leaves the melt under SiO form via reactions: EQU {O}.revreaction.1/2O2.sub.g ( 10) EQU Si.sub.l +1/2O2.sub.g .fwdarw.SiO.sub.g ( 11)
So, clearly, no relationship can be established between carbon generation (reactions (1) to (5)) and oxygen generation (reactions (9) to (11)), especially in the case where reactions (4) and (5) dominate. Consequently, a simple [CO] recording during the crystal pulling cannot be used to monitor carbon and oxygen concentrations in crystals on a real time basis.